Field of the invention and related art statement
The present invention relates to a focus error detecting apparatus which is preferably applied to an apparatus for recording information in an optical information recording medium, such as an optical disc, an optical card and a photomagnetic disc, and/or for reproducing the information recorded in such optical information recording medium.
FIG. 1 is a schematic view showing a conventional focus error detecting apparatus. In the apparatus shown in FIG. 1, a laser beam emitted from a laser diode 1 is made incident upon an optical disc 2 via a lens 3, a polarizing beam splitter 4, a quarter-wave plate 5 and an objective lens 6; and a reflected light beam reflected by the optical disc 2 is introduced to a photodetector 7 via the objective lens 6, the quarter-wave plate 5, the polarizing beam splitter 4, a hologram element 8 and a lens 9.
FIG. 2 is a schematic view depicting a front surface of the photodetector 7. As depicted in FIG. 2, the photodetector 7 comprises a first pair of light receiving portions 10 and 11 for detecting a focusing error signal, a second pair of light receiving portions 12 and 13 for detecting tracking error signal and a third light receiving portion 14 for detecting information reproducing signal; and all of these light receiving portions are arranged in a same plane. The reflected light beam reflected by the optical disc 2 is separated into two light beams one of which is made incident upon the light receiving portion 12 and the other light beam is made incident upon the light receiving portion 13 by the hologram element 8 and the lens 9; and a zero-order diffraction light beam of the reflected light beam is made incident upon the light receiving portion 14; and .+-. first order diffraction light beams are made incident upon the light receiving portions 10 and 11, respectively.
It should be noted that the apparatus is so designed that each of the light beams being made incident upon the light receiving portions 12, 13 and 14 is focused on each light receiving surfaces of the light receiving portions, the plus first order diffraction light beam being made incident upon the light receiving portion 10 is focused on a former position of the light receiving surface of the light receiving portion 10 and the minus first order diffraction light beam being made incident upon the light receiving portion 11 is focused on a rear position of the light receiving surface of the light receiving portion 11. The light receiving portions 10 and 11 comprise three light receiving area 10A, 10B, 10C and 11A, 11B, 11C, respectively.
The focusing error signal is detected from outputs 10a, 10b, 10c of the light receiving area 10A, 10B, 10C and outputs 11a, 11b, 11c of the light receiving area 11A, 11B, 11C by applying the formula of {10a-(10b+10c)-[11a-(11b-11c)}; the tracking error signal is derived from the difference of the outputs of the light receiving portions 12 and 13, and the information reproducing signal is derived from the output of the light receiving portion 14.
FIG. 3 is a schematic view showing another conventional focus error detecting apparatus. In the apparatus shown in FIG. 3, a laser diode 21 and a photodetector 22 are arranged in a same plane; a light beam emitted from the laser diode 21 is introduced to an optical disc 23 via a lens 24, a hologram element 25 and an objective lens 26; and a reflected light beam reflected by the optical disc 23 is made incident upon the photodetector 22 via the objective lens 26, the hologram element 25 and the lens 24.
FIG. 4 is a schematic view representing a front surface of the photodetector 22. In the photodetector 22, are provided a first pair of light receiving portions 27 and 28 for detecting the focusing error signal and the information reproducing signal and a second pair of light receiving portions 29 and 30 for detecting the tracking error signal. The reflected light beam reflected by the optical disc 23 is separated into two light beams one of which is made incident upon the light receiving portion 29 and the other light beam is made incident upon the light receiving portion 30 by the hologram element 25 and the lens 23; and the plus first-order diffraction light beam is separated into two light beams one of which is made incident upon the light receiving portion 27 and the other light beam is made incident upon the light receiving portion 28, respectively, by the hologram element 25 and the lens 24.
Each light beams introduced to the light receiving portions 29 and 30 is focused on each light receiving surfaces of light receiving portions; and the plus first order diffracting light beam introduced to the light receiving portion 27 is focused on a former position of the light receiving portion 27 and the plus first-order diffraction light beam introduced to the light receiving portion 28 is focused on a rear position of the light receiving portion 28. The light receiving portions 27 and 28 comprise three light receiving area 27A, 27B, 27C and 28A, 28B, 28C, respectively.
The focusing error signal is detected from the outputs 27a, 27b, 27c of the light receiving area 27A, 27B, 27C and the outputs 28a, 28b, 28c of the light receiving area 28A, 28B, 28C by applying the formula of {27a-(27b+27c)}-{28a-(28b-28c)}; the information reproducing signal is derived from the sum of the outputs of the light receiving portions 27 and 28; and the tracking error signal is derived from the difference between the outputs of the light receiving portions 29 and 30.
The apparatus shown in FIG. 1 has a advantage that the utilizing efficiency of the light beam is high and thus the focal point can be detected with a high degree of reliability. However, since the laser diode 1 and the photodetector 9 are independently arranged in different planes in this apparatus, the apparatus as a whole becomes large in size.
Contrary, in the apparatus shown in FIG. 3, the laser diode 21 and the photodetector 22 are arranged in a same plane, so that the size of the apparatus as a whole can be made compact. However, in this apparatus, since the focusing error signal is detected by the +first order diffraction light beam only, the utilizing efficiency of the light beam is not so high, and then the focus error can not be detected with a high responsibility.
In order to solve the above-mentioned problem, an apparatus having a construction shown in FIG. 5 can be suggested. In this apparatus, a laser diode 31 and two photodetectors 32, 33 for obtaining the focusing error signal are arranged in a same plane; the light beam emitted from the laser diode 31 is introduced to an optical disc 34 via a hologram element 35 and an objective lens 36; the reflected light beam reflected by the optical disc 34 is made incident upon the hologram element 35 via the objective lens 36 and .+-. first-order diffraction light beams are generated thereby. The .+-. first-order diffraction light beams ar made incident upon the photodetectors 32 and 33, respectively, such that the focusing positions of the diffraction light beams are deviated in front and in rear from the light receiving portions of the photodetectors 32 and 33; and the focusing error signal is derived from the difference between the outputs of the photodiodes 32 and 33. However, in the apparatus shown in FIG. 5, since the laser diode 31 and the photodetectors 32 and 33 are arranged independently, it is necessary to adjust the positions of the photodetectors 32 and 33 with respect to the laser diode 31 in a diffraction direction (x) of the diffraction light beam diffracted by the hologram element 35 and in a direction (y), which is perpendicular to the x direction, in addition to adjusting the position of the photodiodes 32, 33 in an optical axis direction (z). Therefore, the number of position adjusting process increases and the cost for manufacturing the apparatus becomes expensive.
In the apparatus shown in FIG. 5, when the wavelength .lambda. of the laser beam emitted from the laser diode 31 is about 780 nm (.+-.10 nm), the distance between the light incident points of the laser diode 31 and each of the light incident points of the .+-. first-order diffraction light beam being made incident upon the photodetectors 32 and 33 is about 0.4 mm, in order to limit a focusing offset to .+-.0.2 .mu.m or less and limit a variation of focussing responsibility to 20% or less, the positioning accuracy shown in the below-mentioned table is required for the photodiodes 32 and 33 and the hologram element 35.
TABLE ______________________________________ Parameter Tolerance ______________________________________ Photodetectors X.sub.p .+-.100 .mu.m Y.sub.p .+-.5 .mu.m Z.sub.p .+-.10 .mu.m .theta..sub.p .+-.0.5 deg X.sub.d .+-.100 .mu.m Y.sub.d .+-.2 .mu.m Hologram element X.sub.H .+-.90 .mu.m Y.sub.H .+-.90 .mu.m Z.sub.H .+-.100 .mu.m .theta..sub.H .+-.0.5 deg ______________________________________
wherein:
X.sub.P :deviation in x direction of photodiodes with respect to laser diode; PA0 Y.sub.P : deviation in y direction of photodiodes with respect to laser diode; PA0 Z.sub.P : deviation in z direction of photodiodes with respect to laser diode; PA0 .theta..sub.P : rotation deviation of photodiodes; PA0 X.sub.d : deviation in x direction between photodiodes; PA0 Y.sub.d : deviation in y direction between photodiodes; PA0 X.sub.H : deviation in x direction of hologram element; PA0 Y.sub.H : deviation in y direction of hologram element; PA0 Z.sub.H : deviation in z direction of hologram element; PA0 .theta..sub.H : rotation deviation of hologram element
These parameters except Z.sub.P and Z.sub.H are illustrated in FIGS. 6 to 9. It should be noted that the reference X in FIG. 8 represents the distance between the light incident points of the .+-. first-order diffraction light beams being made incident upon the photodetectors 32 and 33. In this case, the distance X is about 0.8 mm.
From the above-mentioned table, it is proved that the tolerances of the parameters Y.sub.P, Y.sub.d and Z.sub.P are very strict when adjusting the position of the photodetectors 32 and 33.
The parameter .theta..sub.H of the hologram element 35 can be used to adjust the parameter Y.sub.P of the photodetectors. However, when adjusting the parameter .theta..sub.H of the hologram element 35, the spots of the .+-. first-order diffraction light beams move on the photodetectors 32, 33 as shown in FIG. 9. Thus, it is possible to adjust the parameter .theta..sub.H such that the one of the spots is positioned on one of the photodetectors 32 and 33, but the other spot can not be introduced on the other photodetector. Similarly, it is used to adjust the deviation Y.sub.d between the photodetectors in the y direction by adjusting the parameter .theta..sub.H of the hologram element 35. But it is not possible to introduce both of the spots of the .+-. first-order diffraction light beams just on the photodiodes 32 and 33, respectively. Further, the parameter X.sub.H of the hologram element 35 can be used to adjust the deviation Z.sub.P of the photodetectors 32 and 33 in the z direction with respect to the laser diode. But, when moving the hologram element 35 is moved in x direction, the movement of the spot of the plus first-order diffraction light beam is different from that of the spot of the minus first-order diffraction light beam, so that the deviations of the photodiodes 32 and 33 in z direction with respect to the laser diode 31 (Z.sub.P) cannot be adjusted.
As explained in above in detail, in the apparatus having such a construction that the laser diode 31 and photodetectors 32 and 33 are independently arranged, and the .+-. first-order diffraction light beams diffracted by the hologram element 35 are received by the photodetectors 32, 33, which are arranged symmetrically with respect to the laser diode 31, it is not possible to adjust the positioning deviations of the photodiodes 32, 33 with respect to the laser diode 31 by using parameters of other element such as hologram element 35. Therefore, the photodiodes 32, 33 and the laser diode 31 must be positioned with a strictly high accuracy; and thus, the number of adjusting process is increases and the cost for manufacturing the apparatus becomes expensive.
Another apparatuses for detecting a focus error are disclosed in Japanese Patent Laid Open Publication Nos. 56-57013 and 63-10325. FIG. 10 is a schematic view representing the conventional apparatus disclosed in these publications. In the apparatus represented in FIG. 10, a light beam emitted from a laser diode 41 is reflected by a semi-transmitting surface 42a of a parallel plate 42 and converged by an objective lens 44 toward an optical disc 43 ; a reflected light beam reflected by the optical disc 43 is transmitted through the semi-transmitting surface 42a of the parallel plate 42 and is reflected by a reflecting surface 42b of the parallel plate 42; thereafter the light beam is made incident upon a photodetector 45 via the semitransmitting surface 42a.
When the reflected light beam reflected by the optical disc 43 is made incident upon the semitransmitting surface 42a from a perspective direction, an astigmatism is introduced into the reflected light beam due to the thickness of the parallel plate 42. In order to detect a variation of the light beam caused by the astigmatism, the photodetector 45 comprises four light receiving area 45A to 45D, as shown in FIG. 11. The focusing error signal and information reproducing signal are detected by the outputs 45a, 45b, 45c, 45d of the light receiving area 45A, 45B, 45C, 45D. That is to say, the focusing error signal is derived from the outputs 45a, 45b, 45c, 45d by applying the formula {(45a+45c)-(45b+45d)}; and the information reproducing signal is derived from the sum of the outputs 45a to 45d of the light receiving area 45A to 45D.
In this apparatus, since the laser diode 41 and the photodetector 45 are arranged separately, it is difficult to make the apparatus compact. Further, an error in size of the parallel plate 42 causes a problem that an optical axis of the reflected light beam being made incident upon the photodetector 45 is deviated from a desired position and thus the spot of the reflected light beam is not positioned on a center of the photodetector 45. That is to say, if there is no error in the thickness of the parallel plate 42, the center of the spot 46 of the reflected light beam is coincident with the intersection center of the dividing lines which divide the photodetector 45 as shown in FIG. 12A. However, if the thickness of the plate 42 is thinner than a given thickness, the center of the spot 46 is deviated from the intersection point of the dividing lines in an upper direction (laser diode 45 side) as shown in FIG. 12B; and if the thickness of the plate is thicker than the given thickness, the center of the spot 46 is deviated from the intersection point of the dividing lines in a lower direction as shown in FIG. 12C.
As stated above, when the optical axis of the reflected light beam is deviated from the desired position due to the error of the thickness of the parallel plate 42, it is necessary to adjust the position of the photodetector 45 with a strictly high accuracy of about 0.5 .mu.m or less. Therefore, there is also the problem that the number of adjusting process increases and the cost for manufacturing the apparatus becomes high.